1. Field of the Invention
The present invention relates generally to an ink-jet printing apparatus and an ink-jet printing method performing printing by ejecting an ink toward a printing medium. More particularly, the invention relates to reduction of variations of density at a boundary between printing scanning regions, so-called banding, and so on.
2. Description of the Related Art
A recording apparatus to be employed as a printing output means of a printer, a copy machine, a facsimile machine and so on, or a recording apparatus to be employed as a printing output device of a composite type electronic device including a computer, a word processor and so on, a work station and so on is constructed for forming an image or the like on a medium to be recorded, such as a paper, a plastic film and the like (hereinafter referred to as a printing medium or paper) on the basis of an image information (including all information including character information and the like to be output). Such recording apparatus may be separated into an ink-jet type, a wire dot type, a thermal type, a laser beam type and so on. Amongst, the ink-jet type recording apparatus (hereinafter referred to as ink-jet recording apparatus) is designed to perform recording by ejecting ink toward the medium to be printed from recording means including a recording head and are advantageous in respect of easiness of increasing of recording density, high speed, superior in silentness, and low cost in comparison with other recording type. On the other hand, in the recent years, needs for color output of color image and so forth is progressively increasing and large number of color ink-jet recording apparatus have been developed.
In such ink-jet recording apparatus, in order to improve recording speed, a recording head, in which a plurality of ink ejection openings and a plurality of liquid passages are integrated is employed as the printing head having a plurality of recording elements integrated as an array (hereinafter referred to as multi-head). Furthermore, for adaptation for color printing, it is typical to provide a plurality of multi-heads.
FIG. 1 shows a construction of a major portion of the apparatus for performing recording (hereinafter referred to printing) employing the multi-heads set forth above. In FIG. 1, the reference numerals 101 denote ink-jet cartridges. Each of these is constructed with an ink tank storing black, cyan, magenta and yellow ink and the multi-head 102 corresponding to each color of the inks. FIG. 2 is a diagrammatic illustration of the ejection openings (hereinafter also referred to as nozzles) arranged in the multi-head 102 as viewed in z direction in FIG. 1. In FIG. 2, the reference numeral 201 denotes ejection openings arranged on each multi-head 102.
Referring again to FIG. 1, the reference numeral 103 denotes a paper feeding roller which is driven to rotate in a direction shown by arrow with gripping a printing paper P together with an auxiliary roller 104 for feeding the printing paper P in y direction. On the other hand, the reference numeral 105 denotes a pair of paper feeding rollers for feeding the printing paper. A pair of rollers 105 are designed to rotate with gripping the printing paper P in a similar manner to that of the rollers 103 and 104. By driving the rollers 105 at lower rotation speed than that of the paper feeding roller 103 appropriately, a tension can be applied to the printing paper appropriately. The reference numeral 106 denotes a carriage supporting the four ink-jet cartridges and scans those ink-jet cartridges according to progress of printing. While printing is not performed, the carriage 106 is placed in stand-by state at a home position h as illustrated by broken line while recovery process of ejection of the multi-head 102 is performed.
The carriage 106 at the home position h at initiation of printing, is moved in the x direction, responsive to a command for initiation of printing, so that printing for a width L on the paper by means of n in number of nozzles 201 of the multi-head 102 is performed. Once printing of data up to the end portion of the paper surface is completed, the carriage is returned to the original home position to again perform printing in x direction. After finishing of first print and before initiation of second print, the paper feeding roller 103 is rotated in the direction of arrow to perform paper feeding in y direction in the extent of the width L. As set forth above, by repeating printing for the multi-head width L and paper feeding per one scan of the carriage 106, printing for one page, for example, is completed.
In such ink-jet type printing apparatus, different from printing in a monochrome printer which prints only characters, for example, various printing characteristics, such as a color development characteristics, a gradation characteristics, uniformity of density and so on are required upon printing a graphic image. Particularly, for uniformity of density, it has been known that variations per nozzle due to slight tolerance caused during multi-head manufacturing process becomes perceptible as variations of the ejection amount and/or ejecting direction of each nozzle upon actual printing and finally can be a cause of degradation of a printing quality as density variations of the printing image.
Particular example will be explained with reference to FIGS. 3A, 3B, 3C and 4A, 4B, 4C. In FIG. 3A, the reference numeral 31 denotes a multi-head, in which eight nozzles 32 are arranged. The reference numeral 33 denotes an ink droplet ejected from each nozzle 32. Ideally, as shown in FIG. 3A, it is desirable to eject the ink in the same direction with substantially equal ejection amount. If ejection is performed in this manner, uniform size of dots are formed on the paper surface as shown in FIG. 3B to obtain a uniform image having no density fluctuation over the entire area (FIG. 3C).
However, in practice, due to variations of the nozzles as set forth above, if printing is performed in the same manner as that shown in FIGS. 3A to 3C, dots as shown in FIG. 4B are formed on the paper surface due to variations of size and direction of the ink droplets ejected from the nozzles as shown in FIG. 4A. In the example shown in FIGS. 4A to 4C, blank paper portions along with the head scanning direction which do not satisfy 100% of area factor, appear cyclically. Also, a black banding due to excessive overlap of dots, or, in the alternative, white banding as seen at the center of FIG. 4B can be caused. The image consisted of dots formed in such condition has density distribution shown in FIG. 4C with respect to arrangement direction of the nozzles and results in recognition as density variations.
On the other hand, different from the example shown in FIGS. 4A to 4C, it has conventionally known a banding due to variations of paper feeding amount.
As a solution for density variations or joint variations, in Japanese Patent Application Laying-open No. 60-107975 (1985), the following method has been disclosed in a monochrome ink-jet printing apparatus. A method will be briefly explained with reference to FIGS. 5A, 5B, 5C and 6A, 6B and 6C. In the shown method, scanning by the multi-head 31 is repeated for three times in order to complete printing in the shown printing region (FIG. 5A), in which the half of the printing region corresponding to each four nozzles are completely printed by twice of scanning (hereinafter also referred to as pass). Namely, in this case, eight nozzles of the multi-head 31 are separated into two groups of upper four nozzles and lower four nozzles. The dots to be printed by one scan of one nozzle corresponds to the predetermined image data thinned by substantially half in accordance with a predetermined pattern. Then, by performing complementary scanning according to respective patterns through twice of scanning, printing of the region corresponding to respective of upper and lower four nozzles can be completed. Foregoing manner of printing method will be hereinafter referred to as divided printing method.
By employing the divided printing method as set forth above, since influence of variations in each individual nozzle for the printed image in each region can be reduced into substantially half even when the multi-head the same as that shown in FIGS. 4A to 4C is employed, the printed image becomes as illustrated in FIG. 5B so that a black banding or a white banding is not perceptible as that shown in FIG. 4B. Accordingly, density variations can also be reduced significantly in comparison with the case shown in FIGS. 4A to 4C, as shown in FIG. 5C.
When printing is performed as set forth above, the image data is divided into a predetermined pattern in mutually complementary manner in the first scan and the second scan. In such case, it is typical to arrange the image data (thinning pattern) in lattice fashion per every other pixels in vertical and horizontal direction, as shown in FIG. 6C. Accordingly, in a unit printing region (here, a region corresponding to four nozzles), printing can be completed by the first scan, in which one lattice or checker pattern is printed, and the second scan, in which the other lattice or checker pattern (inverted lattice or checker pattern) complementary to one lattice or checker pattern.
FIGS. 6A, 6B and 6C illustrate how a predetermined region is printed by printing of one lattice pattern and the other lattice or checker pattern are performed.
In FIGS. 6A, 6B and 6C, in the first scan, one lattice or checker pattern is printed as shown in FIG. 6A by means of the lower four nozzles. Next, in the second scan, after performing paper feeding in a magnitude corresponding to four pixels (1/2 of head length), the other lattice pattern is printed by all nozzles (FIG. 6B). Furthermore, in the third scan, after paper feeding is performed in a magnitude corresponding to four pixels (1/2 of head length), one lattice pattern is again printed (FIG. 6C). In this manner, the paper feeding in a unit of four pixels, and printing of one lattice pattern and the other lattice pattern are performed alternately to complete printing of each printing regions of four pixels per each scan.
As set forth above, by completing printing by different two kinds of nozzles in the same region, high quality image free from density variations can be obtained.
On the other hand, as a technical task in the ink-jet printing apparatus, it has been known a problem associating with absorbency and evaporation characteristics of the ink on the printing paper, that irregularity of the dot shape and undesirable dots connection by deposition of the ink can be caused. Also, in the color ink-jet printing apparatus, in which a plurality of mutually different colors of inks are sequentially deposited in overlapping manner or at adjacent positions, a problem of degradation of printing quality due to undesirable bleeding or admixing of inks has been known. The conventional divided printing method set forth above may achieve an effect of improvement of the foregoing problem for smaller ink ejection amount per one scan. On the other hand, a multi-pass printing method, in which one image region is simply divided into a plurality of times of printing scan without performing particular paper feeding control as in the divided printing method, to perform printing according to thinning pattern in each scan or to perform printing in sequential order of colors, has been proposed as a measure for the problem set forth above.
In the divided printing method or the multi-pass printing method, a period required for completing printing of the image region except for the boundary of each scanning region corresponds to the number of printing scan cycles necessary for completing the image. However, in the region adjacent the boundary (hereinafter referred to as boundary portion), since printing in the first scan of the scanning region adjacent the boundary portion should influence the boundary portion, it takes a period to finally determine the density of the image in the boundary portion longer than that in the other scanning region by the extent corresponding to one scan. Namely, typically, the dot forming each pixel is formed in a size beyond the size of the pixel and dots of the adjacent pixels are partly overlapped. Accordingly, in the pixels of the boundary portion, density of these pixels can be determined only after initiation of printing in adjacent scanning region.
In the foregoing case, dots in respective boundary portions of adjacent scanning regions are formed with a time difference of at least one scan on the printing paper with each other. Thus, there arises a problem of causing a banding as density variations due to difference of penetration and fixing condition of the ink forming respective dots on the printing medium due to the time difference.
FIGS. 7A to 7D are diagrammatic illustrations showing a manner of deposition and fixing of the ink on the printing paper, which shows that fixing of the later deposited ink should be influenced depending upon degree of fixing of the ink deposited in advance.
Namely, the ink is ejected in a certain scan as shown in FIG. 7A. Then, the ink thus ejected is deposited on the printing paper P in a condition as represented by the ink portion shown by a black solid portion as shown in FIG. 7B. When the ink is ejected in the next scan in a condition where the ink deposited in the former scan is not sufficiently fixed as shown in FIG. 7C, later ejected ink may locally penetrate into the lower side of the former deposited ink to cause difference of the fixing condition as illustrated by the hatched portion in FIG. 7D. Such overlapping portion of the dots formed with the time difference is caused in the boundary portion to form the banding providing different appearance from other region. Particularly, in the solid print region to be printed at high printing duty, overlapping of the adjacent dots can be caused to make the density variations further significant to cause the banding.